Bad news and dangerous materials in our makeup isn't exactly new. You just have to look back at Queen Elizabeth's propensity for piling on the lead face paint to know that humans have been risking their lives, unknowingly, for beauty for many thousands of years. (Lead still pops up occasionally in lipsticks, by the way.) All chemicals are not, by definition, bad: that's severely reductive and a misunderstanding of how science works. But there's always the problem of unexpected side effects, where a genius product turns out to have a killer set of difficulties nobody anticipated at the time. Look at CFCs: they were used as awesome refrigerants and solvents for years, until somebody discovered the nasty ozone layer problem. Whoops.

1. Parabens

What It's Used For: Parabens are also preservatives, and work as anti-bacterial agents to keep cosmetic products fresh.

Why People Worry About Them: Parabens have attracted a lot of negative attention because studies have found six different parabens inside breast cancer tumours in female patients, in concentrations that seemed to tie in with the use of cosmetic products. At the time, scientists were careful to point out that they didn't know that parabens caused cancer, just that they were present.

But a new study in 2015 found that parabens also prompted the swift growth of breast cancer cells in a lab environment. The tie between parabens and breast cancer may be strengthening, but scientists still point out that they don't yet have a smoking gun, and that most people have very low amounts of parabens in their daily lives.

2. Sulfates

What It's Used For: Sulfates are preservatives, and are most commonly found in hair products and bleaches, along with some skin products.

Why People Worry About Them: Sulfates have been a part of preserving food and substances since ancient Greek times, but that doesn't mean it's always been a good idea. The good news? Sulfates are essentially fine, if you don't have a sensitivity to them. If you do, however, the consequences can be extremely severe.

Sensitivity is rare but not extremely uncommon: over one percent of the population probably has a sensitivity, and you're far more likely to have it if you're already asthmatic. Scientists have identified a huge swathe of potential reactions, including "dermatitis, urticaria, flushing, hypotension, abdominal pain and diarrhea [and] life-threatening anaphylactic and asthmatic reactions". So if you have asthma or think you've got a sensitivity, steer clear of sulfate-containing products.

5. Mercury

Why People Worry About It: Remember the Mad Hatter? People have known about mercury poisoning for a long time; hatters were historically stereotyped as mad because they underwent serious mercury exposure while making headgear.